1. Field of the Invention
The present invention concerns a device for generating aerodynamic forces for the active control of buffet response of tail surfaces on an aircraft.
2. Description of the Related Art
The tail surfaces of a high-speed fighter aircraft operating at a high angle of attack are enveloped by the wake of turbulent vortex flow generated upstream by leading edges of the wings. This unsteady flow field produces dynamic loads and results in severe buffet responses on such tail surfaces. For instance, on the F-18 aircraft, buffet responses as high as .+-.450 g's have been measured on the vertical tail tips during high angle-of-attack flight. Such vibrations have resulted in premature fatigue failures of the structure and costly efforts to re-engineer and strengthen the structure.
Schemes for alleviating problems of aircraft buffet at high angles of attack can be characterized as either passive or active.
Passive methods include, for example, strengthening and stiffening the aircraft structure itself; modifying the vortex flow field by aerodynamic devices such as wing leading edge extensions (LEX), fences, and wing leading edge blowing or sucking; and employing damping devices like viscoelastic layer constrainers, tuned vibration absorbers, and tuned dampers.
Active methods of buffet alleviation employ an active feedback control system to suppress the aircraft response due to buffet excitation. In this approach, buffet response signals are processed by a control law and fed back as input to a force generator which reduces the buffeting. Examples of proposed active feedback control concepts for suppressing buffet response include: aircraft flight control surfaces, such as the rudder; an auxiliary aerodynamic surface, such as a tip-mounted vane; and so-called "smart structures" involving piezoelectric actuators bonded to the tail.
Modifying the aircraft flight control system is disadvantageous because coupling a buffet suppression system with either the rudder or another primary flight control surface introduces safety risks and added complexity. Also, the buffet suppression system may possibly be rendered ineffective either because of the location of the flight control surface or because of high-frequency band width limitations on actuators for the flight control surface.
Tip-mounted vanes are disadvantageous because they add drag and may alter aircraft stability and handling qualities.
Smart structures involve emerging, but unproven technology. For example, piezoelectric actuators may require a large electrical power input to be effective. The change in the shape of the aircraft structure controlled by the piezoelectric actuator is small, thus limiting its effectiveness in producing aerodynamic control forces for buffet suppression.